Solid-state imaging device and method of manufacturing the same

ABSTRACT

In a high density solid-state imaging device, of four charge transfer electrodes formed on a semiconductor substrate via a gate insulating film, a first electrode, a fourth electrode, and a part of a second electrode are made of a first conductive film, and a third electrode and the remaining portion of the second electrode are made of a second conductive film. In the second electrode, the first conductive film is joined to the second conductive film. An oxidation film formed by thermally oxidizing the first conductive film isolates the first electrode from the second electrode, the second electrode from the third electrode, and the third electrode from the fourth electrode. The end of the second conductive film is formed so as to locate on the oxidation film on the first conductive film.

BACKGROUND OF THE INVENTION

This invention relates to a solid-state imaging device using afour-phase charge-coupled device (CCD) having four gate electrodes for asingle photodiode and to a method of manufacturing the solid-stateimaging device.

A CCD is a semiconductor device that has a structure where plural phasecharge transfer electrodes are formed on a semiconductor channel regionformed in a semiconductor substrate via a thin gate insulating film of athickness ranging from 0.1 to 0.2 μm, for example, and that transfers acharge signal by applying pulse voltages to the plural phase chargetransfer electrodes, respectively, and changing the potential of thechannel region under the electrodes.

FIG. 11 shows the structure of one cell of a conventional four-phaseCCD. FIG. 11A is a top view of the cell. FIG. 11B is a sectional viewtaken along line XIB--XIB in FIG. 11A. FIG. 11C is a sectional viewtaken along line XIC--XIC of FIG. 11A.

As shown in FIG. 11A, a CCD solid-state imaging device is composed of aphotodiode 11 constituting a light-receiving section (pixel section) andfour phase charge transfer electrodes formed in the direction of, forexample, column of the photodiode 11. On the photodiode 11, the chargetransfer electrodes are not formed. Between photodiodes 11 and, forexample, in a region in the direction of row, charge transfer electrodesare formed via a gate insulating film 2 above a semiconductor substrate1, thereby forming a charge transfer region as shown in FIG. 11B, whereits sectional view is shown. The charge transfer electrodes formed in aregion in the direction of, for example, column between photodiodes 11constitute an interconnection layer region for supplying voltage to theindividual charge transfer electrodes in the charge transfer region asshown in FIG. 11C, where its sectional view is shown. Although notshown, on the regions except for the light-receiving section, a shadingfilm is formed of a metal layer film made of, for example, Al.

The charge transfer electrodes shown in the figure are formed of threelayers of polysilicon film: a first charge transfer electrode is made ofa first-layer polysilicon film; a second and fourth-layer chargetransfer electrodes are made of a second-layer polysilicon film, and athird charge transfer electrodes is made of a third-layer polysiliconfilm. The individual charge transfer electrodes are isolated from eachother by an oxide film. The oxide film is formed by patterning thepolysilicon film of each layer using, for example, lithography andetching techniques and by thereafter oxidizing the patterned polysiliconlayer.

To form an oxide film for isolating the individual charge transferelectrodes from each other as described above, it is necessary topattern each polysilicon film separately. This approach thereforerequires mating margins for patterning in processing each layer, whichmakes it difficult to subminiaturize cells. Furthermore, to secure themating margins, the area of a photodiode 11 is reduced, leading to theproblem of lowering the sensitivity.

As shown in FIG. 11C, in the interconnection layer region, three layersof polysilicon films are stacked one on top of another, so the stepheight between the interconnection layer region and the photodioderegion 11 is large. In forming a shading film, this prevents the shadingfilm from being formed sufficiently over the step portion and thereforelight may enter the regions other than the photodiode 11, causing anerroneous signal.

In contrast, there is a method of thickening a shading film so that theshading film may be formed sufficiently even at the step portion. Withthis method, however, when the shading film on the photodiode region 11is etched away, it is difficult to etch the film away sufficiently.Because the shading film is formed thicker in the periphery of thephotodiode region 11, the amount of light reaching the photodiode 11 issmaller, lowering the sensitivity of the solid-state imaging device. Forthis reason, it is desirable that the thickness of the shading filmshould not be made thicker.

Moreover, since it is necessary to form and process three layers ofpolysilicon film, the processes are long and complex.

As described above, with the conventional solid-state imaging device andmethod of manufacturing the device, since the charge transfer electrodesare composed of three layers of polysilicon films, the step heightbetween a region where three layers of polysilicon films are stacked oneon top of another, such as an interconnection layer region, and thephotodiode region is large, which prevents a shading film from coveringthe step portion sufficiently, resulting in the problem of permittinglight to enter the regions other than the photodiode, thus causing anerroneous signal.

Furthermore, because the three layers of polysilicon films must bepatterned separately, it is necessary to secure mating margins forpatterning, leading to the problem of making it difficult tosubminiaturize cells.

In addition, because it is necessary to process the three layers ofpolysilicon films, this results in the problem of making themanufacturing processes long and complex.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to provide a high-densitysolid-state imaging device which can suppress the generation oferroneous signals by reducing the step height between theinterconnection region and the charge transfer electrode region andphotodiode region and whose manufacturing processes are simple, and amethod of manufacturing the solid-state imaging device.

The foregoing object is accomplished by providing a solid-state imagingdevice comprising: a plurality of light-receiving elements formed on asemiconductor substrate; and a set of four charge transfer electrodesthat are formed via a gate insulating film in the regions between thelight-receiving elements and that are applied with four different pulsesignals, the set of four charge transfer electrodes being arrangedrepeatedly, wherein a first charge transfer electrode, a fourth chargetransfer electrode, and part of a second charge transfer electrode inthe set of charge transfer electrodes are made of a first conductivefilm, a third charge transfer electrode and the remaining portion of thesecond charge transfer electrode in the set of charge transferelectrodes are made of second conductive film, the first conductive filmis joined to the second conductive film in the second charge transferelectrode, an oxide film formed by thermally oxidizing the firstconductive film isolates the first charge transfer electrode from thesecond charge transfer electrode, the second charge transfer electrodefrom the third charge transfer electrode, and the third charge transferelectrode from the fourth charge transfer electrode, and the end of thesecond conductive film is formed so as to locate on the oxide film onthe first conductive film.

The solid-state imaging device may further comprise a first conductivematerial portion formed of the second conductive film joined to thesidewall of the first charge transfer electrode in a set of chargetransfer electrodes adjacent to the fourth charge transfer electrode;and a second conductive material portion formed of the second conductivefilm joined to the sidewall of the fourth charge transfer electrode in aset of charge transfer electrodes adjacent to the first charge transferelectrode.

In the solid-state imaging device, the first and second conductive filmsmay be made of a polysilicon film.

The foregoing object is also accomplished by providing a method ofmanufacturing solid-state imaging devices, comprising: the step offorming a gate insulating film on a semiconductor substrate; the step offorming a first conductive film on the gate insulating film; the step ofremoving the first conductive film in part of a second charge transferelectrode region and in a third charge transfer electrode region so thatthe first conductive film may have a strip pattern; the step ofthermally oxidizing the surface of the remaining first conductive film;the step of removing, on the first charge transfer electrode regionside, part of a thermal oxidation film formed in the thermal oxidationprocess on the first conductive film constituting part of the secondcharge transfer electrode; the step of removing the thermal oxidationfilm in the region between the fourth charge transfer electrode and thefirst charge transfer electrode in a set of charge transfer electrodesadjacent to the fourth charge transfer electrode and the firstconductive film so that they may have a strip pattern; the step offorming a second conductive film; the step of forming a first to fourthstrip charge transfer electrodes by removing the second conductive filmusing as a mask a first resist film having an opening on part of theinsulating film parallel with and partially overlapping with the firststrip conductive film and remaining on the second charge transferelectrode and on the region between the fourth charge transfer electrodeand the first charge transfer electrode in a set of charge transferelectrodes adjacent to the fourth charge transfer electrode; and

the step of removing the second conductive film, the thermal oxidationfilm, and the first conductive film using as a mask a second resist filmhaving an opening on a light-receiving element region on the first tofourth strip charge transfer electrodes.

In the method of manufacturing solid-state imaging devices, the step offorming a first to fourth strip charge transfer electrodes by removingthe second conductive film may include the step of etching the secondconductive film using anisotropic etching techniques.

As described above, with the solid-state imaging device of the presentinvention, because the four charge transfer electrodes to which fourdifferent pulse signals are applied are made of the first and secondlayer conductive films, the step heights due to the charge transferelectrodes are reduced as compared with a conventional equivalent wherethe charge transfer electrodes were made of three layers of conductivefilms. This makes it possible to form a shading film at a high coveringrate, which helps prevent light from entering the regions other than thelight-receiving region, thus avoiding the generation of erroneoussignals.

Because the thermal oxidation film of the first conductive film, whichis generally excellent in insulation, isolates the first charge transferelectrode from the second charge transfer electrode, the second chargetransfer electrode from the third charge transfer electrode, and thethird charge transfer electrode from the fourth charge transferelectrode, it is possible to isolate the charge transfer electrodes fromeach other reliably.

Because the distance between the charge transfer electrodes on thechannel region through which charges are transferred is determined bythe thickness of the thermal oxidation film, the distance between thecharge transfer electrodes can be made constant. This makes it easy toset the impurity concentration distribution in the channel region.

Furthermore, because the distance between the charge transfer electrodescan be made smaller than the minimum dimension the lithography processwill allow, the transfer efficiency of charges can be improved.

Since the end of the second conductive film is formed so as to locate onthe oxide film on the first conductive film, there is no need ofsecuring mating margins in the lithography process, which helps reducethe area of cells and form a highly integrated solid-state imagingdevice. When the cell area is not reduced, the area of thelight-receiving element can be increased, improving the sensitivity ofthe solid-state imaging device.

With the solid-state imaging device of the present invention where thefirst conductive material portion is formed of the second conductivefilm joined to the sidewall of the first charge transfer electrode in aset of charge transfer electrodes adjacent to the fourth charge transferelectrode and the second conductive material portion is formed of thesecond conductive film joined to the sidewall of the fourth chargetransfer electrode in a set of charge transfer electrodes adjacent tothe first charge transfer electrode, because the first conductivematerial portion is joined to the fourth charge transfer electrode andthe second conductive material portion is joined to the first chargetransfer electrode, these conductive material portions function as thefourth or first charge transfer electrode. Namely, this produces thesame effect of the fourth or first charge transfer electrode beingextended. This enables the conductive material portions to shorten thedistance between adjacent sets of charge transfer electrodes, therebymaking the distance smaller than the minimum dimension the lithographyprocess will allow. This helps improve the transfer efficiency ofcharges.

With the solid-state imaging device of the present invention where thefirst and second conductive films are formed of polysilicon films, aninsulating film excellent in insulation can be formed by thermallyoxidizing the polysilicon film constituting the first conductive film.

With the method of manufacturing solid-state imaging devices accordingto the present invention, the first to fourth charge transfer electrodesare formed of the first and second conductive films formed on thesemiconductor substrate via the gate insulating film. By forming thecharge transfer electrodes of two layers of conductive films this way,the step heights due to the charge transfer electrodes can be reduced ascompared with a conventional equivalent where the charge transferelectrodes were made of three layers of conductive films.

Because the first conductive film is processed so as to have a strippattern and the second conductive film is formed after the processedfirst conductive film is thermally oxidized, the first conductive filmcan be isolated from the second conductive film by the thermal oxidationfilm of the first conductive film.

Because the first conductive film is isolated from the second conductivefilm by the thermal oxidation film of the first conductive film and thesecond conductive film is processed so as to overlap with the firstconductive film, the distance between the charge transfer electrodes onthe channel region is determined by the thickness of the thermaloxidation film, which enables the distance to always remain constant.Moreover, the distance can be made smaller than the minimum dimensionthe lithography process will allow.

The length of each of the charge transfer electrodes on the channelregion is influenced only by the lithography process in processing thefirst conductive film, but not by the other lithography processes. Thisprevents the length of each charge transfer electrode from varying withthe mating accuracy of the lithography processes. As a result, thecharacteristic of transferring charges can be stabilized.

After the first to fourth charge transfer electrodes are formed byprocessing the first and second conductive films into a strip pattern,the second conductive film, thermal oxidation film, and first conductivefilm are removed using as a mask a resist film having an opening on thelight-receiving element region, which eliminates the necessity for themating margins for patterning, unlike a conventional manufacturingmethod where each conductive film on the light-receiving element regionwas removed by separate patterning. This reduces the area of cells,helping producing a highly integrated solid-state imaging device. Whenthe cell area is not reduced, the area of the light-receiving elementcan be increased, improving the sensitivity of the solid-state imagingdevice.

With a conventional manufacturing method, the first to fourth chargetransfer electrodes were formed of three layers of conductive films.Therefore, it was difficult to make an opening on the light-receivingelement region by etching the three layers of conductive films and theinsulating films between the layers using the same resist film as a maskafter processing the three layers of conductive films into a strippattern, because the resistance of the resist film might not besufficient. In contrast, with the manufacturing method of the presentinvention, because two layers of conductive films are used to form thefirst to fourth charge transfer electrodes, an opening can be madeeasily on the light-receiving region by etching the first and secondconductive films and the insulating film between them using the sameresist as a mask.

Furthermore, with the method of manufacturing solid-state imagingdevices according to the present invention, in the step of removing thesecond conductive film to form the first to fourth strip charge transferelectrodes, since anisotropic etching is performed using as a mask thefirst resist film having an opening on part of the insulating filmparallel to and partly overlapping with the first strip conductive filmand remaining on the second charge transfer electrode and on the regionbetween the fourth charge transfer electrode and the first chargetransfer electrode in a set of charge transfer electrodes adjacent tothe fourth charge transfer electrode, the second conductive film remainson the sidewalls of the fourth charge transfer electrode and firstcharge transfer electrode in the region between the fourth chargetransfer electrode and the first charge transfer electrode adjacent tothe fourth charge transfer electrode, thereby enabling a conductivematerial portion to be formed of the remaining second conductive film.Therefore, the conductive material portion makes the distance betweenthe fourth charge transfer electrode and the first charge transferelectrode smaller than the minimum dimension the lithography processwill allow. As a result, the transfer efficiency of charges can beimproved.

Additional objects advantages of the invention will be set forth in thedescription which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIGS. 1A to 1C are sectional views and top view of a solid-state imagingdevice of an embodiment according to the present invention;

FIGS. 2A and 2B are a sectional view and top view to help explain amethod of manufacturing the solid-state imaging device according to theembodiment of the present invention;

FIGS. 3A and 3B are a sectional view and top view to help explain themethod of the manufacturing solid-state imaging device according to theembodiment of the present invention;

FIGS. 4A and 4B are a sectional view and top view to help explain themethod of the manufacturing solid-state imaging device according to theembodiment of the present invention;

FIGS. 5A and 5B are a sectional view and top view to help explain themethod of the manufacturing solid-state imaging device according to theembodiment of the present invention;

FIGS. 6A and 6B are a sectional view and top view to help explain themethod of the manufacturing solid-state imaging device according to theembodiment of the present invention;

FIGS. 7A and 7B are a sectional view and top view to help explain themethod of the manufacturing solid-state imaging device according to theembodiment of the present invention;

FIGS. 8A and 8B are a sectional view and top view to help explain themethod of the manufacturing solid-state imaging device according to theembodiment of the present invention;

FIGS. 9A and 9B are a sectional view and top view to help explain themethod of the manufacturing solid-state imaging device according to theembodiment of the present invention;

FIGS. 10A and 10B are a sectional view and top view to help explain themethod of the manufacturing solid-state imaging device according to theembodiment of the present invention; and

FIGS. 11A to 11C are a top view and sectional views of a conventionalsolid-state imaging device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, referring to the accompanying drawings, an embodiment ofthe present invention will be explained.

FIGS. 1A to 1C are sectional views and top view of a solid-state imagingdevice according to an embodiment of the present invention. FIG. 1C is atop view of the solid-state imaging device of the present embodiment.FIG. 1A is a sectional view of the solid-state imaging device takenalong line 1A--1A in FIG. 1C. FIG. 1B is a sectional view of thesolid-state imaging device taken along line 1B--1B in FIG. 1C.

In FIGS. 1A to 1C, a shading film is not shown as in FIGS. 11A to 11Cshowing a conventional equivalent.

The CCD solid-state imaging device according to the embodiment, like theconventional one, comprises a photodiode 11 constituting alight-receiving section (pixel section) and four phase charge transferelectrodes formed in the direction of, for example, column of thephotodiode 11. On the photodiode 11, the charge transfer electrodes arenot formed. Between photodiodes 11 and, for example, in a region in thedirection of row, charge transfer electrodes are formed via a gateinsulating film 2 above a semiconductor substrate 1, thereby forming acharge transfer region as shown in FIG. 1B, where its sectional view isshown. The charge transfer electrodes formed in a region in thedirection of, for example, column between photo-diodes 11 constitute aninterconnection layer region for supplying voltage to the individualcharge transfer electrodes in the charge transfer region.

The CCD solid-state imaging device of the embodiment is provided with afirst to fourth charge transfer electrodes as a conventional equivalent,but differs from the conventional equivalent whose four phase chargetransfer electrodes were made of three layers of polysilicon films inthat the four phase charge transfer electrodes are composed of twolayers of polysilicon films. As shown in FIG. 1B, the first chargetransfer electrode is composed of a first layer polysilicon film portion3a and a sidewall polysilicon film 8a formed by forcing the second layerpolysilicon film to remain on the sidewall of the polysilicon film 3a.The second charge transfer electrode is composed of the firstpolysilicon film portion 3b and the second layer polysilicon filmportion 8b formed so as to connect to the polysilicon film portion 3b.The third charge transfer electrode is made of the second polysiliconfilm portion 8c. The fourth charge transfer electrode is composed of thefirst polysilicon film portion 3d and a sidewall polysilicon film 8dformed by forcing the second layer polysilicon film to remain on thesidewall of the polysilicon film 3d.

As described above, with the embodiment, because the four phase chargetransfer electrodes are composed of two layers of polysilicon films, thestep heights due to the charge transfer electrodes are reduced ascompared with the conventional equivalent whose charge transferelectrodes were composed of three layers of polysilicon films. Thismakes it possible to form a shading film at a great covering rate on thesidewall surface of an opening made to expose the photodiode region 11,which prevents light from entering the regions other than the photodioderegion 11 and therefore avoids the generation of erroneous signals dueto such entering light.

Furthermore, because the distance between the photodiode region 11 and alens for gathering rays of light on the photodiode 11 is shortened, theloss of light is decreased, improving the sensitivity of the solid-stateimaging device.

Moreover, with the embodiment, for pairs of adjacent charge transferelectrodes (i.e., the first charge transfer electrode portion 3a andsecond charge transfer electrode portion 8b, the second charge transferelectrode 3b and third charge transfer electrode 8c, and the thirdcharge transfer electrode 8c and fourth charge transfer electrodeportion 3d) in each cell, one charge transfer electrode is made of thefirst layer polysilicon film and the other is made of the second layerpolysilicon film so that one may overlap with the other. One chargetransfer electrode is isolated from the other by a thermal oxidationfilm 5 formed by thermally oxidizing the first polysilicon film.

This makes it possible to make the distance between the charge transferelectrodes at the interface with the gate insulating film 2 smaller thanthe minimum dimension the lithographic process will allow. By shorteningthe distance between the adjacent charge transfer electrodes like this,the transfer efficiency of charges in the channel region can beimproved. Because the distance is determined by the thickness of thethermal oxidation film 5, the distance can be made constant. This makesit possible to easily set the impurity concentration distribution in thechannel region.

Furthermore, since the insulating properties of the thermal oxidationfilm are better than those of an insulating film formed by a depositionmethod, it is possible to prevent short circuit from taking placebetween the charge transfer electrodes.

Still furthermore, the present embodiment is characterized in that onthe sidewalls of the first layer polysilicon film portions 3d and 3aconstituting the fourth charge transfer electrode and first chargetransfer electrode, respectively, the sidewall polysilicon films 8d and8a are formed of the second layer polysilicon film. With this structure,the distance between the fourth charge transfer electrode and firstcharge transfer electrode adjacent to each other can be made smallerthan the minimum dimension the lithographic process will allow.Consequently, as described earlier, the transfer efficiency of chargesin the channel region can be improved.

Next, a method of manufacturing such a solid-state imaging device willbe described by reference to FIGS. 2A to 10B. Although FIGS. 1A to 1Cshow four cells, FIGS. 2A to 10B show a single cell. FIGS. 2B, 3B, 4B,5B, 6B, 7B, 8B, 9B, and 10B are top views. FIGS. 2A, 3A, 4A, 5A, 6A, 7A,8A, 9A, and 10A are sectional views of the semiconductor structure takenalong line IIA--IIA, IIIA--IIIA, IVA--IVA, VA--VA, VIA--VIA, VIIA--VIIA,VIIIA--VIIIA, IXA--IXA, and XA--XA of FIGS. 2B, 3B, 4B, 5B, 6B, 7B, 8B,9B, and 10B, respectively. The first to fourth charge transfer electroderegions constituting four phase charge transfer electrodes are arrangedfrom left to right in each of those figures.

First, on a semiconductor substrate 1, a gate insulating film 2 made ofan insulating film, such as an oxide film or a nitride film, is formed.Then, on the insulating film, a first layer polysilicon film 3 of, forexample, 400 to 500 nm thick is formed. Next, on the polysilicon film 3,a resist film 4 with a strip pattern as shown in FIG. 2B is formed. Withthe resist film 4 as a mask, the polysilicon film 3 is etched (FIG. 2A).Unlike a conventional solid-state imaging device manufacturing method inwhich the polysilicon film on the photodiode region 11 is removed inprocessing the polysilicon film of each layer, the present embodimentperforms processing only in the direction of the interconnection regionwithout removing the polysilicon film 3 on the photodiode region 11.Moreover, a first charge transfer electrode 3a, a part 3b of a secondcharge transfer electrode, and a fourth charge transfer electrode 3d areformed of the first layer polysilicon film 3.

Next, as shown in FIGS. 3A and 3B, on the surfaces of the processedpolysilicon films 3a, 3b, and 3d, an oxide film 5 of, for example, about0.2 μm thick is formed by, for example, thermal oxidation.

Thereafter, as shown in FIGS. 4A and 4B, a resist film 6 having, forexample, a pattern as shown in FIG. 4B is formed so that of the oxidefilm 5 formed on the sidewall surface of the second charge transferelectrode 3b, that part facing the first charge transfer electrode 3amay be exposed. Next, with the resist film 6 as a mask, the oxide film 5formed on the sidewall of the second charge transfer electrode 3b isremoved by an etching solution, such as NH₄ F (FIG. 4B).

Then, to isolate the fourth charge transfer electrode from the firstcharge transfer electrode, the two adjacent to each other, a resist film7 having a strip pattern as shown in FIG. 5B is formed. With this resistfilm as a mask, part of the oxide film 5 on the polysilicon films 3a and3d and part of the polysilicon films 3a and 3d are removed by etchingtechniques, such as anisotropic etching. Here, it is desirable that inetching the polysilicon films 3a and 3d, the etching speed of thepolysilicon film 3 should be about 20 times as fast as the etching speedof the insulating film constituting the gate insulating film 2, such asan oxide film or a nitride film. With the desirable speed, it ispossible to protect the semiconductor substrate 1 with the gateinsulating film 2 in etching the polysilicon film 3, thereby preventingthe semiconductor substrate 1 from being damaged.

Thereafter, as shown in FIG. 6A, a second layer polysilicon film 8 of,for example, 400 nm to 500 nm thick is formed. Then, as shown in FIG.6B, on the polysilicon film 8, part of the oxide film 5 on the firstlayer polysilicon film 3b constituting the second charge transferelectrode is exposed to form a resist film 9 having such a pattern asexposes the space between the first charge transfer electrode 3a and thefourth charge transfer electrode 3d completely.

Next, with the resist film 9 as a mask, the second layer polysiliconfilm 8 is etched by anisotropic etching techniques to form a part 8b ofthe second charge transfer electrode and a third charge transferelectrode 8c. By setting the amount of etching to the extent that, forexample, a polysilicon film whose thickness is about 1.2 to 1.5 timesthat of the second polysilicon film 8 is etched, the second layerpolysilicon films 8a and 8d are allowed to remain on the sidewalls ofthe first charge transfer electrode 3a and fourth charge transferelectrode 3d as shown in FIGS. 7A and 7B. Specifically, the distancebetween the fourth charge transfer electrode and first charge transferelectrode of adjacent cells is determined by the distance between thepolysilicon films 8d and 8a remaining on the sidewalls.

Thereafter, to remove the first layer polysilicon film 3 and secondlayer polysilicon film 8 on the photodiode region 11, a resist film 9having a pattern as shown in FIG. 8B is formed. Then, with the resistfilm 9 as a mask, the second layer polysilicon film 8 is removed by, forexample, anisotropic etching techniques. It is desirable that in theetching, the etching speed of the polysilicon film should be more than20 times as fast as the etching speed of the insulating filmconstituting the gate insulating film 2, such as an oxide film or anitride film.

Then, with the resist film 9 as a mask, for example, the thermaloxidation film 5 formed on the first layer polysilicon film is removedby etching techniques using, for example, NH₄ F, as shown in FIGS. 9Aand 9B. It is desirable that in the etching, the etching speed of theoxide film should be more than 50 times as fast as the etching speed ofthe insulating film constituting the gate insulating film 2, such as anitride film.

Then, with the resist film 9 as a mask, the portions exposed in thefirst layer polysilicon film 3 are removed by, for example, the samemethod as removing the second layer polysilicon film 8. Thereafter, theresist film 9 is removed to expose the photodiode region 11 as shownFIGS. 10A and 10B. FIGS. 1A to 1C show the state at that time.

Thereafter, a thermal oxidation film 8 is formed on, for example, thesecond layer polysilicon film 8. Then, a metal film of, for example, Alis formed so as to cover the first and second layer polysilicon films.Then, the metal film on the photodiode region is removed and the firstto fourth charge transfer electrodes are covered with the metal film,thereby forming a shading film. This completes the CCD solid-stateimaging device.

With the solid-state imaging device manufacturing method of the presentembodiment, because the charge transfer electrodes are formed of twolayers of polysilicon films, the step heights due to the charge transferelectrodes are reduced as compared with the conventional method wherethree layers of polysilicon films were used to form four phase chargetransfer electrodes, which improves the covering rate of the shadingfilm. This prevents light from entering the portions other than thephotodiode and therefore suppresses the generation of erroneous signals.

Furthermore, the present embodiment is characterized by forming each oftwo layers of polysilicon films composing charge transfer electrodesinto a strip pattern and then removing the two layers of polysiliconfilms on the photodiode region 11 in a single lithography process. Thiseliminates the necessity of securing a mating margin for each pattern,unlike a case where each layer of polysilicon film is processed usinglithographic techniques. As a result, it is possible to subminiaturizecells. When the cell area is set constant, the area of the photodioderegion 11 can be increased, improving the sensitivity of the solid-stateimaging device.

With the conventional method of forming charge transfer electrodes fromthree layers of polysilicon films, it was difficult to etch the threelayers of polysilicon films and two layers of insulating film betweenthe films using the same resist film formed in the lithography processas a mask, because the resistance of the resist film to etching was notsufficient. With the present embodiment, however, only two layers ofpolysilicon films and a single layer of insulating film have to beetched. Therefore, the resist film withstands the etching sufficiently,enabling an opening to be made on the photodiode region 11 in a singlelithography process.

Furthermore, with the present embodiment, although the second layerpolysilicon film is patterned on the patterned first layer polysiliconfilm, it is not patterned on the gate insulating film 2. As a result,the length of the first to fourth charge transfer electrodes at theinterface with the gate insulating film 2 is influenced only by thepatterning of the first layer polysilicon film. As described above,because the length of each of the four phase charge transfer electrodesis determined in a single lithography process, it is possible to preventthe length of the charge transfer electrodes from changing due to themisalignment of patterning in processing the individual charge transferelectrodes. Because the length of each charge transfer electrode canalways be made constant, the setting of the impurity concentrationdistribution in the channel region can be done easily. Moreover, becausethe length of each charge transfer electrode is not influenced by themisalignment of patterning, a burden to the lithography process can bereduced in processing the second layer polysilicon film.

With the present embodiment, after the first layer polysilicon film isprocessed and then the processed first layer polysilicon film isoxidized thermally to form a thermal oxidation film 5, the second layerpolysilicon film is formed. Therefore, the distance between adjacentcharge transfer electrodes in a single cell is determined by thethickness of the thermal oxidation film. The distance between theportions in contact with the interface with the gate insulating film 2is particularly made smaller than the minimum dimension the lithographyprocess will allow. This makes it possible to improve the transferefficiency of charges.

Furthermore, with the present embodiment, after the first layerpolysilicon film is processed to isolate cells from each other, thesecond layer polysilicon film is formed. Thereafter, the second layerpolysilicon film is processed. Here, by processing the second layerpolysilicon film by anisotropic etching techniques, it is possible tocause the second layer polysilicon film to remain on the sidewall of thepreviously processed first layer polysilicon film. In this way, thedistance between cells can be made smaller than the minimum dimensionthe lithography process will allow. This makes it possible to improvethe transfer efficiency of charges.

Still furthermore, by using a stacked layer film including, for example,a nitride film as an insulating film constituting the gate insulatingfilm 2, the semiconductor substrate 1 can be protected in removing theoxide film on the polysilicon film in the photodiode region 11.

Similarly, when the polysilicon film in the photodiode region 11 isremoved, the etching speed of the polysilicon film is made more than 20times as fast as the etching speed of the gate insulating film 2,thereby enabling the gate insulating film 2 to protect the semiconductorsubstrate 1 in the photodiode region 11.

As described until now, with the solid-state imaging device of thepresent invention and the method of manufacturing the device, thegeneration of erroneous signals can be suppressed by reducing the stepheight between the charge transfer region and the photodiode region,which makes it easy to realize a high density solid-state imagingdevice.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

It is claimed:
 1. A solid-state imaging device comprising:a plurality oflight-receiving elements formed on a semiconductor substrate; and a setof four charge transfer electrodes that are formed via a gate insulatingfilm in the regions between the light-receiving elements and that areapplied with four different pulse signals, the set of four chargetransfer electrodes being arranged repeatedly, wherein a first chargetransfer electrode and a fourth charge transfer electrode in said set ofcharge transfer electrodes are made of a first conductive film, a secondcharge transfer electrode in said set of charge transfer electrodes aremade of said first conductive film and a second conductive film, and athird charge transfer electrode in said set of charge transferelectrodes is made of said second conductive film, an oxide film formedby thermally oxidizing said first conductive film isolates said firstcharge transfer electrode from said second charge transfer electrode,said second charge transfer electrode from said third charge transferelectrode, and said third charge transfer electrode from said fourthcharge transfer electrode, and the end of said second conductive film islocated on the said oxide film on said first conductive film.
 2. Asolid-state imaging device according to claim 1, wherein said first andsecond conductive films are composed of a polysilicon film.
 3. Asolid-state imaging device comprising:a plurality of light-receivingelements formed on a semiconductor substrate; a set of four chargetransfer electrodes that are formed via a gate insulating film in theregions between the light-receiving elements and that are applied withfour different pulse signals, the set of foul charge transfer electrodesbeing arranged repeatedly, wherein a first charge transfer electrode, afourth charge transfer electrode, and part of a second charge transferelectrode in said set of charge transfer electrodes are made of a firstconductive film, a third charge transfer electrode and the remainingportion of said second charge transfer electrode in said set of chargetransfer electrodes are made of a second conductive film, said firstconductive film is joined to said second conductive film in said secondcharge transfer electrode, an oxide film formed by thermally oxidizingsaid first conductive film isolates said first charge transfer electrodefrom said second charge transfer electrode, said second charge transferelectrode from said third charge transfer electrode, and said thirdcharge transfer electrode from said fourth charge transfer electrode,the end of said second conductive film is located on the said oxide filmon said first conductive film; a first conductive material portionformed of said second conductive film joined to the sidewall of thefirst charge transfer electrode in a set of charge transfer electrodesadjacent to said fourth charge transfer electrode; and a secondconductive material portion formed of said second conductive film joinedto the sidewall of the fourth charge transfer electrode in a set ofcharge transfer electrodes adjacent to said first charge transferelectrode.
 4. A solid-state imaging device according to claim 3, whereinsaid first and second conductive films are composed of a polysiliconfilm.